The problems of the origin and structure of chondrules in stony meteorites

590.51
FI
v.2:2l
cop. 3

UN" -RSITYOF
ILLlK^iS LIBRARY

AT URBANA-CHAMPAIGN
NATURAL HIST. SURVEY

FIELDIANA • GEOLOGY

Published by
CHICAGO NATURAL HISTORY MUSEUM

Volume 10 September 24, 1957 No. 31

THE PROBLEMS OF THE ORIGIN AND

STRUCTURE OF CHONDRULES

IN STONY METEORITES

Sharat Kumar Roy

Chief Curator, Department of Geology

Chondrules are spheroidal aggregates of one or more silicates,
which occur in about 90 per cent of all stony meteorites. In form,
manner of crystallization, texture and structure, they are like no
other spheroidal bodies observed in terrestrial rocks.

The study of chondrules — how they were formed and the alter-
ations they have undergone subsequent to their formation — is not
an end in itself; it has significant bearing on related problems. As
a constituent part of chondritic meteorites, chondrules reflect the
conditions under which the chondritic meteorites themselves were
formed, and, by extension, the conditions under which meteorites
of all types were formed. Chondrules bring to notice a type of rock
which exists in the solar system and which, it may be presumed, has
the sort of composition likely to be found in the substances in the
interior of the earth. No discussion that relates to the origin of
meteorites or to the material constituting the interior of the earth
can be complete that does not take into account the nature and
origin of chondrules. This matter has not received the attention it
deserves.

MINERALOGY OF CHONDRULES

The general mineralogy of the chondrules is fairly well estab-
lished. It is essentially the same as the matrix in which it occurs.
In order of their relative abundance the constituent minerals forming
chondrules are: olivine; pyroxene; plagioclase feldspar, ranging from
anorthite to oliogoclase; glass; nickel-iron; iron sulphides, both
pyrrhotite and the variety troilite; and chromite. There are others,
commonly referred to as "minor constituents," such as apatite
(merrillite) and manganapatite. Because of their minute size and

Library of Congress Catalog Card Number: 57-H.622
No. 825 383

384 FIELDIANA: GEOLOGY, VOLUME 10

because they occur co-mixed with dust-like aggregations of other
minerals, it is often well nigh impossible to isolate or identify most
of these with any degree of certainty, optically or otherwise.

Olivine and pyroxene are the principal constituents. Feldspar
occurs rarely, but there is maskelynite, a mineral, the exact nature
of which is yet to be determined. Some regard it as a meta-
morphosed product resulting from re-fusion of feldspar; others
as a distinct mineral allied to leucite. Chondrules containing
plagioclase in excess of olivine and manifesting alternating barred
structures are best seen in the Dharamshala meteorite. A high
temperature feldspar (probably a variety of albite) has been seen
in the Walters meteorite (Walters: Glass, Roy and Henderson, MS.).
The mineral shows considerable optical abnormality. Biaxial nega-
tive, 2V = 40° to 60°. Birefringence = 0.007. Glass, clear or stained
or clouded with inclusions, is a common constituent of chondrules.
Glass containing microlites of enstatite also occurs.

Nickel-iron occurs as blebs, as small interstitial grains, and as
rims surrounding some chondrules. Rounded bodies resembling
chondrules and composed wholly of nickel-iron are uncommon, but
they do occur. Such bodies may or may not contain grains of iron
sulphide. The latter mineral, however, occurs in chondrules com-
posed chiefly of silicates. Chromite in microscopic grains occurs as
inclusions usually near the surface of the chondrules. The presence
of inclusions of iron, iron sulphide, and chromite in large quantities
may render the chondrules completely opaque. On the whole, the
constituent minerals of the chondrules are often the same as those
of the ground mass in which they are embedded. This may be con-
sidered as a significant factor in the conditions which have brought
about the formation of the chondrules.

The metalliferous portions — the iron-nickel and the iron sulphide
of the chondrules and of the ground mass of the stony meteorites —
are the same as those of the iron meteorites. This may be regarded
as another significant feature.

OCCURRENCE, TEXTURE AND STRUCTURE OF CHONDRULES

The silicate content of the stony meteorites diminishes from the
silicate phase toward the iron phase; thus, I favor the interpretation
that the meteorites, whether iron, iron-stone, or stone, were derived
from a mass that had density stratification and a metallic core
surrounded by silicate shells of decreasing density. Even if the
meteorites are fragments of more than one cosmic mass, it would

ROY: CHONDRULES IN STONY METEORITES 385

still seem from the meteorites known to us that those separate
masses had similar structure, therefore the same cooling history and
the same mode of crystallization and crystal settling. That the iron
and stone meteorites exhibit, respectively, well-formed and hasty
crystallization, further indicates that the iron was covered by the
silicates. In the cooling of such a mass under a silicate mantle, the
temperature of the interior would remain sufficiently high to keep
the iron viscous long enough to favor slow cooling and the growth
of well-formed crystals; the silicate exterior, being exposed, would
naturally cool more rapidly and thus crystallize more rapidly.
Thus, it would seem that the stone and iron meteorites were born
of the same parent, rather than of different ones.

Texturally and structurally the chondrules in the same meteorite
show great variations in shape, size, and manner of crystallization.
The last may vary from densely crypto-crystalline through a great
many intermediate forms to holo-crystalline. The center of crystal-
lization also varies; it may be eccentric or multiple. The variations
of these features are so great that no attempt here has been made to
describe each individually. Instead, a few representative photo-
micrographs (figs. 164-175) have been selected to illustrate some of
the diversities and attending complexities that are presented.

PREVIOUS WORK AND PRESENT STATUS

In 1915, my predecessor in this Museum, the late Dr. 0. C.
Farrington, stated: "The conditions which have brought about the
formation of chondri are not well understood, though the question
has been much discussed and various hypotheses have been sug-
gested." (Farrington, 1915, p. 108.) The substance of this state-
ment is still materially correct.

Soon after the introduction of the petrographic microscope,
Reichenbach (1860) announced that chondrules are older small
meteorites enclosed in younger and bigger ones, "Meteoriten en
Meteoriten." A number of investigators since then have expressed
widely different views. To summarize briefly: (1) chondrules are
fused drops of "fiery rain" (Sorby, 1864, 1877); (2) chondrules are
fragments of pre-existing meteorites, which have become rounded
by oscillation and attrition (Tschermak, 1895); (3) chondrules are
products of a special phase of magmatic segregation, formed in place
as a result of rapid, arrested crystallization in a molten mass (Bre-
zina, 1885); (4) chondrules originated from dispersal of a silicate

386 FIELD lANA: GEOLOGY, VOLUME 10

melt in a hot atmosphere, the resultant drops crystallizing from the
outside inward (Wahl, 1911); (5) chondrules are metamorphosed
garnets — garnets converted to enstatite (Fermor, 1938); and (6)
chondrules were produced by the cooling of liquid silicates, which
fell as a molten rain during a collision of a small asteroid with
a larger one (Urey and Craig, 1953).

The diverse and conflicting views cited here indicate that the
problem is still an obscure one. The principal reason seems to be
that many of the hypotheses proposed were based upon examination
of a limited number of chondrites — access to a greater number was
obviously difficult or impossible. Another reason is that little or
no attention was paid to the importance of the order of crystal-
lization of minerals from solution. Some of the views presented
were inferences drawn from chemical analyses or from literature that
itself contained no concrete information. In a field of inquiry of
this sort, where direct contact with the objects of research is possible and
essential, inferential hypotheses are not likely to meet the requirements
of acceptance.

My own tentative view is one that, in some respects, reflects
the concepts of Brezina (1885) and Wahl (1911). The occurrences of
pyroxene chondrules enclosed by olivine in situ, seem to me irrefu-
table evidence that they were formed in place, as products of mag-
matic separation. The anomalous relationships of various com-
ponents, so marked in chondritic meteorites, can be the result of
subsequent deformation and metamorphism. Practically all chon-
dritic meteorites — if not all — have undergone a certain degree of
metamorphism, and some have undergone repeated metamorphism
(Paragould: Roy and Wyant, 1955; Walters: Glass, Roy and Hender-
son, MS; see also Wahl, 1952). I have no definite explanation of
the eccentric or multiple centers of crystallization or the occurrence
of astonishing variations in texture in chondrules of identical com-
position, often within the narrow space of a fraction of a square
centimeter.

SUGGESTED GENERAL PLAN OF STUDY AND PROCEDURE

Chondrules are of igneous origin; they were subjected to laws
similar to those which govern the formation of terrestrial igneous
rocks. With this in mind, and recalling that the relationships
between components of a rock cannot be divorced from its physical
history, studies should begin with thin sections, and in some cases,
polished surfaces. The features to be noted in order of importance

ROY: CHONDRULES IN STONY METEORITES 387

are: the order in which the different minerals have appeared; the
degrees of metamorphism; textural and structural variations; and the
distribution and interrelationships of the various components of the
chondrules. Detailed knowledge of these features is indispensable;
it may reveal the original environment of the chondrules and
provide the information necessary for building an acceptable theory.
Thermometamorphism and brecciation have played an important
role in producing the deviations in chondrules from the norm, but
these later changes and adjustments can be traced, once the original
environment has been established. Color microphotographs of thin
sections, in ordinary light and between crossed nicols, and black
and white photographs of some of the polished surfaces, are of the
utmost importance in this study, both for the interpretation of the
features observed under the microscope and as a permanent reference
for comparison and discussion of controversial points.

The problem is more one of petrology than one of analytical
chemistry. It deals with forms and features, the mode of formation
of which cannot be satisfactorily interpreted alone in the light of
elements and compounds present. Their distribution and inter-
relationships should be seen and examined in polished and thin
sections and the necessary interpretations should then be made.

REFERENCES

Berwerth, F.

1901. iJber die Struktur der chondritischen Meteorsteine. Centralbl. fiir
Mineral., Geo!, und Palaeont., no. 21, pp. 641-647. Stuttgart.

Brezina, a.

1885. Die Meteoritensammlung des K. K. mineralogischen Hofkabinettes in
Wien. Jahrb. K. K. Geol. Reichsan., 35. Vienna.

Cohen, E.

1903. Struktur der Steinmeteorite Meteoritenkunde. Part 2. Stuttgart.

Daubree, a.

1867. Contribution a I'anatomie des meteorites. Compt. Rend. Acad. Sci.,
65. Paris.

Farrington, 0. C.

1915. Meteorites, their structure, composition, and terrestrial relations.
Lakeside Press, Chicago,

Fermor, L. L.

1938. Garnets and their role in nature. Ind. Assoc. Adv. Sci., special pub.,
no. 6, pp. 87-91.

Merrill, G. P.

1920. On chondrules and chondritic structure in meteorites. Proc. Nat. Acad.
Sci., Washington, 6, no. 8.

388 FIELDIANA: GEOLOGY, VOLUME 10

Meunier, Stanislaus

1869. Recherches sur la composition et la structure des meteorites. Ann.
Chim. et Phys., 18.

Reichenbach, C.

1860. Meteoriten en Meteoriten. Ann. Phys. und Chem. von J. C. Poggen-
dorff, 111.

Renard, a.

1899. Recherches sur le mode de structure des meteorites chondritiques.
Bull. Acad. Roy. Belgique, 31.

Roy, S. K., and Wyant, R. K.

1955. The Paragould meteorite. Fieldiana, Geol., 10, no. 23, pp. 283-304.

SORBY, H. C.

1864. On the microscopical structure of meteorites. Proc. Roy. Soc. London.
1877. On the structure and origin of meteorites. Nature, April 5. London.

TSCHERMAK, G.

1895. Die mikroscopische Beschaffenheit der Meteoriten erlautert durch
photographische Abbildungen. Stuttgart.

Urey, H., and Craig, H.

1953. The composition of the stone meteorites and the origin of the meteorites.
Geochim. et Cosmochim. Acta, 4, nos. 1-2.

Wahl, W.

1911. Beitrage zur Chemie der Meteoriten. Zeitschr. fiir anorgan. und allg.

Chem., 69.
1952. The brecciated stony meteorites and meteorites containing foreign

fragments. Geochim. et Cosmochim. Acta, 2, pp. 91-117.

Fig. 164. Mezo-Madaras meteorite; Transylvania. Polymict brecciated
gray hypersthene-chondrite; X 40.

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389

Fig. 166. Mezo-Madaras meteorite;
gray hypersthene-chondrite; X 40.

Transylvania. Polymict brecciated

390

Fig. 167. Dharamshala meteorite; Kangra District, Punjab, India. Inter-
mediate hypersthene-chondrite; X 40.

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Fig. 168. Kesen meteorite; Iwate, Honshu, Japan. Spherical hypersthene-
chondrite; X 40.

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Fig. 169. Parnallee meteorite; Madura District, Madras, India. Polymict
brecciated veined gray hypersthene-chondrite; X 40.

393

Fig. 170. Beaver Creek meteorite; West Kootenay District, British Colum-
bia. Crystalline spherical bronzite-chondrite; X 40.

Fig. 171. Weston meteorite; Fairfield
brecciated spherical chondrite; X 40.

County, Connecticut. Polymict

394

Fig. 172. Knyahinya meteorite; Nagy-Bereszna, Czeclioslovakia. Polymict
brecciated gray hypersthene-chondrite; X 40.

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Fig. 173. Pultusk meteorite; Warsaw, Poland. Veined gray bronzite-chon-
drite; X 40.

395

Fig. 174. Ensisheim meteorite; Alsace, France. Polymict brecciated crystal-
line hypersthene-chondrite; X 40.

Fig. 175. Ausson meteorite; Haute Garonne, France. Spherical hypersthene-
chondrite; X 40.

396